Apparatus and method for electric field telemetry employing component upper and lower housings in a well pipestring

ABSTRACT

An apparatus for borehole electric-field telemetry that comprises a source of modulated voltage or current, at least one axially non-conductive collar connected between pipe sections in a pipe string, and a system of insulated wireline components providing electrical connections, insulated from drilling fluids, between the ends of the one or more aforementioned insulated collars in the pipe string, to transmit the voltage to current.

This application is a continuation-in-part of Ser. No. 08/707,270, filedSep. 3, 1996, now U.S. Pat. No. 5,883,516 which claims priority fromprovisional application Ser. No. 60/024,794, filed Jul. 31, 1996.

BACK GROUND OF THE INVENTION

The prior art for electromagnetic drillstring telemetry is based uponinductive (toroidal) or direct coupling of a source signal carrying thedownhole sensor information to the drillstring and surroundingformation. Toroidal coupled systems induce a modulated electric currenton the drillstring by means of electromagnetic coupling between a(primary) toroidal coil encircling a conductive mandrel connected to thedrillstring, and a secondary coil comprising the drillstring, andsurrounding formation. The modulated current, which is induced in thesecondary, flows along the drillstring and drilling fluid, and throughthe formation in a pattern, which is governed by the electricalconductivity(s) of the drillstring and drilling fluid, and surroundingformation. The flow of current on the drillstring and through theformation is measured by a receiving apparatus at the surface.

The receiving apparatus is either inductively coupled to the modulatedcurrent through a transformer or directly coupled by sensing thepotential difference (voltage) produced by the flow of modulated currentbetween electrodes “grounded” at the surface. A previous patent (U.S.Pat. No. 4,181,014 to Zuvela et al) describes several means of signalreception using sub-surface electrodes connected to the surface byinsulated conductors. (See also U.S. Pat. No. 4,980,682 to Klein et al).

The operation of the inductively coupled (toroidal) downholetransmitter-receiver (transceiver) is enhanced by insulating gaps in thedownhole transceiver sub-assembly to isolate the toroidal primary coilfrom the surrounding drill collar (which would otherwise provide adirect short to the secondary, if it were not electrically isolated).The toroidal-inducing coil encircles an electrically conducting mandrel,which is mechanically and electrically connected to the upper and lowersections of drillstring. The toroidal sub-assembly and associatedelectronics are designed to provide impedance matching between thesource circuitry and the load of the drillstring-formation circuit (U.S.Pat. No. 4,496,174 to McDonald et al, 1985).

In the prior art, the source impedance may be matched with the loadusing matching transformers (U.S. Pat. No. 2,389,241 to Silverman, 1944;U.S. Pat. No. 4,691,203 to Rubin, 1987). Matching transformers andassociated complex electrical circuitry are employed to match theimpedance of the downhole sub-assembly electronics to the very lowimpedance associated with the small gaps necessary to maintain themechanical stability of the downhole transceiver sub-assembly. One ofthe herein inventors has previously patented an apparatus forelectromechanical impedance matching (U.S. Pat. No. 5,130,706 to VanSteenwyk, 1992).

Transformer coupled electric-field telemetry systems require that thesignal information be transmitted by various forms of modulation of acarrier signal. Pulse modulated systems have been described (U.S. Pat.No. 3,046,474 to Arps, 1962; U.S. Pat. No. 4,015,234 to Krebs, 1977);but these systems have required the generation of a very high-voltagepulse by means of capacitor discharge to overcome the poor impedancematch between the downhole transmitter and the drillstring-formationload impedance.

More recently, a low-voltage, low-impedance, current generator has beendescribed (U.S. Pat. No. 5,270,703 to Guest). It should be noted thatnone of these methods for coupling a pulse to the drillstring-formationpath are suited to a talk-down capability. See also U.S. Pat. No.4,684,946 to Geoservice.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus to improve theeffectiveness of electric-field borehole telemetry. A direct-coupledelectromagnetic telemetry system is provided in which the downholesource drives a modulated electric current directly into the undergroundformation by means of a modulated voltage or current applied across anelectrically insulating gap created in the drillstring by one or moregap sub-assemblies.

Another aspect of the invention is directed to the use of insulatingdrill collars and wireline components, to match the downhole impedanceof electric signal transmitter circuitry to the electrical impedance ofthe surrounding drilling fluids and geologic formations. By means ofthis aspect of the invention, downhole power requirements can besignificantly reduced.

Another feature of the invention is the use of the downhole electricfields generated by the telemetry apparatus for formation resistivityand induced polarization measurements. By using insulating drill collarsand wireline components to vary transmitter and receiver electrodespacing and configuration, many of the methods of surface resistivityand induced polarization available to surface geophysics can be deployedon the drillstring, in conjunction with a downhole electric fieldtelemetry system.

The invention provides a method and apparatus to configure an insulatinggap in a drillstring or borehole casing, so as to enable the generationor detection of electric fields on the surface of the drillstring orborehole casing. The method can be used in the transmission of downholemeasurements and drilling parameters from the drillstring to thesurface, the transmission of control signals from the surface to a pointon the drillstring, and the evaluation of resistivity and inducedpolarization response of the formation surrounding the drillstring,formation at the bit, or formation surrounding a cased borehole.

These and other objects and advantages of the invention, as well as thedetails of an illustrative embodiment, will be more fully understoodfrom the following specification and drawings, in which:

DRAWING DESCRIPTION

FIG. 1a shows elements of the invention in block diagram form;

FIG. 1b is a section showing details of the apparatus incorporating theinvention;

FIGS. 2a, 2 b and 2 c show the basic components of the invention inthree possible configurations; FIG. 2a shows the invention configuredwith a single insulating gap; FIG. 2b shows the invention configuredwith the gap positioned uphole of a high resistivity rock layer; FIG. 2cshows the invention configured with two gaps;

FIG. 3 shows an equivalent circuit diagram of the transmission path usedby the invention for downhole telemetry and formation evaluation;

FIG. 4 shows details of the bottom hole assembly for a two-gapconfiguration of the invention;

FIG. 5 shows the invention configured for azimuthal resistivity-at-bitmeasurements;

FIG. 6 shows the invention configured for formation resistivity andinduced polarization response measurements above a motor that drives adrill bit;

FIG. 7 shows the invention configured for azimuthal resistivity andinduced polarization evaluation in the formation adjacent to theborehole;

FIG. 8 is a more detailed view showing component in a drillstring;

FIG. 9 is a section showing details of switching and sensor modules;

FIG. 10 is a block diagram;

FIG. 11 is a section showing adaptation to use with well casing;

FIG. 12 is a section showing use of multiple wirelines;

FIG. 13 shows details of insulative gap construction;

FIG. 14 shows use of a well fluid pressure responsive switch;

FIG. 15 shows use of multiple receiver electrodes;

FIG. 16 shows target detection by means of the invention;

FIG. 17 is another schematic elevation showing apparatus in a pipestring incorporating the invention; and

FIG. 18 is a schematic showing of the use of multiple surfaceelectrodes.

DETAILED DESCRIPTION

The mechanical limitations imposed by the prior art of toroidal coupledborehole telemetry systems, and the difficulties in matching thedrillstring-formation impedance of a short-gap, direct-coupled systemare addressed by the present invention. By providing insulated drillcollars or gap sub-assemblies used in conjunction with electric currentsupplying components and circuits, the invention provides direct coupledimpedance matching, optimum location of the transmission gap in complexgeologic systems, and the integration of formation evaluationgeo-steering, and downhole telemetry, in a single system.

In certain embodiments of the invention, a direct coupled impedancematch, or near match, to the drillstring-formation transmission path isprovided. By proper selection of one or more insulated drill collars orgap sub-assemblies and conventional drill collars, the drillstring isconfigured to present an electrical impedance match between the downholeelectric-field telemetry system and the surrounding formation. Aninsulated wireline may connect upper and lower sub-assemblies forcompleting an electrical circuit comprised of the upper drillstring,power source, wireline, bottom hole assembly, and formation.

A block diagram of the invention is shown in FIG. 1a. A downholetransceiver 100 transmits at 101 either drilling parameters or theresults of formation evaluation measurements to a transceiver 102 at thesurface, or receives signals from a surface transmitter for powermanagement or other control requirements. Note transducers or sensors103, 103 a, and 104 supplying data to the transreceiver. The sameinstrumentation is used for both downhole telemetry and evaluation offormation resistivity and induced polarization (IP) response. Notetransmission line 105 from 102 to 100.

FIG. 1b shows the invention in a measurement-while-drilling (MWD)application. A bent sub-assembly means 302 in the drillstring providesdirectional control for the drilling operations. Voltage applicationapparatus is shown in the string and includes battery 24, insulatedwireline 305, connected at connections 314 and 315 to upper and lowerinstrument housings 311 and 312, which house components, such asbatteries, sensors and switching apparatus. Voltage or current isapplied by electrical contact means 306 and 304 to the drillstring, andthen to the formation. A borehole drill motor 313 is shown in the stringabove the drill bit 316. Upper extent of the string is indicated at 22,and the borehole appears at 22 a, in formation 22 b. A circuitry housingappears at 307. Surface equipment appears at 22 c.

FIGS. 2a, 2 b and 2 c illustrate three possible configurations of thesystem used as a means of downhole electric-field telemetry. In eachconfiguration, a voltage is impressed across an insulated drill collar1, between upper and lower steel drillstring sections 4 and 5, anddrives an electric current through the earth 2. In configuration of FIG.2a, a power source 3 is connected across an upper section 4 of thedrillstring, and a lower section 5 of the drillstring, as by wirelinecomponents 6 and a signal source (modulator) indicated as a switch 7,which opens and closes as a function of data to be transmitted, as via apath defined by the drillstring 4 and 5, and the formation 2. Sections 4and 5 are typically metallic (steel), and collar 1 is in series with 4and 5.

Signals are detected at the surface of the earth by a receiver 8, whichmeasures the voltage produced by the downhole transmitter, as betweentwo electrodes associated with 8 at the surface. Receiver 8 is in a line8 a connected between the upper end of the string 4 and 9 a, and a probe9 into the earth. Note the possible connection 9 b to the steel casingin the borehole. In the configuration shown, one electrode comprises anelectrical attachment to the drillstring, and the other electrode 9 isconnected directly to the earth.

In FIG. 2b, the insulating section 1 of the drillstring is positionedabove the level of a high resistive layer 10 of the formation throughwhich wireline components extend, thus permitting the transmission ofdownhole information through an insulating geologic formation. Noteconnection at 6 a of line 6 to string section 5 a extending below 10,and connection at 6 b to string section 4 b above 1. The drillstringsections 4 b and 5 a consist of steel. Borehole casing is indicated at 4a.

In FIG. 2c, multiple metallic sections 4 b, 4 c, 5 a, and 5 c of thedrillstring are interconnected by insulated sections or collars 1 and 1a. An electrical line 6 interconnects 4 b and 5 a to provide animpedance match and to extend the effective length of the insulatinggap. Other elements remain as shown in FIG. 2b. Current flow in theformation appears at 400 and 401.

An alternate means of telemetry from a downhole location to the surfaceis implemented by modulating the impedance of the entire assembly asmeasured from surface connections 9 and 9 a. A downhole means foralternately electrically connecting and disconnecting portions of thedrillstring is provided by using an appropriately positioned gap or gaps1 in the drillstring sections electrically connected by insulatedwireline components 6 and a switching means 7. In this method, the onlyelectrical power required for this means of downhole telemetry is thatrequired for the operation of the electric switch, thus eliminating theneed for downhole power source 3.

FIG. 3 shows an electrical circuit equivalent of the drillstring-earthtransmission path. The FIG. 3 elements are defined as follows:

17 a very large resistance of the “gap”, i.e., insulated drill collar 1

17′ resistance of metallic drillstring section 4 above 17 a 17″resistance of metallic drillstring section 5 below 17 a

14 battery 3

15 internal impedance of battery

16 resistance of wireline 6

C₁ upper end connection of wireline 6 to drillstring upper section 4

C₂ lower end connection of wireline 6 to drillstring lower section 5

21 resistance of current path at earth surface

18 electrical resistance of drilling mud (between drillstring and earthbore) between C₁ and C₂ levels

19″ electrical impedance of the formation proximate to the boreholeabove level of C₁ and upper end of drillstring

20 effective capacitance of the formation proximate to the borehole mudabove level of C, and upper end of drillstring

e₁ current between C₁ drilling mud

e₂ current between drilling mud and C₂

19′ effective electrical impedance of earth formation between electrode9 and lower section 5 of drillstring

20′ effective capacitance of earth formation between electrode 9 andlower section 5 of drillstring

V₁ measured voltage between upper end of drillstring (and drilling mud),and probe 9.

Note that voltage difference e₁−e₂ is maintained by current flow i_(g)across the gap 17 a. The voltage across the gap is determined largely bythe downhole source voltage at 14, the internal resistance 15 of thesource 14 and wireline 16, and the resistance 18 of the fluids (mud) inthe annulus surrounding the gap sub-assembly. The voltage across the gapdrives a current i_(e) into the earth 2. This flow of current at thesurface produces a voltage drop (V₁) across the resistance 21 of theearth at the surface. The voltage V₁ is measured by the receiverelectronics.

Mechanical detail of a two-gap form of the downhole assembly portion ofthe invention is shown in FIG. 4. The bottom hole assembly is eithermounted above a downhole motor 34 or one or more drill collars. Theupper metallic drillstring section 22 is electrically connected to anupper electrical power source, here represented by a battery 24, as viaconnection 24 a, housing 23, and centralizer bowed spring 23 a engagingthe string bore. Insulated wireline 26, connected to the battery,extends from the lower end of the upper sub-assembly downwardly throughone or more insulting drill collars 27 and 29, and one or moreintermediate, conventional, metallic drill collars 28, to a lowercontrol sub-assembly 31, and a sensor sub-assembly 33.

A drive for the switch 30, in series with line 26, is shown at 30 a. Thedrive is modulated by the output of sensor 33. Line 26 electricallyconnects at 32 to the housing 31, connected to conductive spring 23 b,which electrically engages the bore of lower drillstring section 22 b.The sensor sub-assembly may be located above the motor 34, as shown, orin an instrumentation mandrel (bit box) directly above the bit. Motor 34drives (rotates) drill bit 35.

Reference is now made to FIG. 5. In addition to downhole telemetry, theinvention provides a means for evaluation of resistivity and inducedpolarization (IP) response at the bit, in the formation surrounding thedrillstring or in the formation surrounding a cased borehole. Bygenerating an electric field in the surrounding medium, i.e., formation,and with multiple current or voltage-sensing electrodes placed on thedrillstring, at the bit, or on the casing of a cased borehole, theresistivity and IP response of the surrounding medium can be measured.

To evaluate formation resistivity and IP response at and directly aheadof the bit, a voltage pulse waveform, or a set of selected frequencies,is applied across an impedance matched insulated gap or gaps in thedrillstring and drill collars configured as shown in FIG. 5. The bulkresistivity of the formation surrounding the insulated gap, drill collaror motor, bit-box, and bit can then be determined by well known datareduction methods for geophysical interpretation of formationresistivity and IP response. The resistivity at the bit is analyticallyseparated from the bulk resistivity surrounding the bottom hole assemblyby noting that, as the bottom hole assembly passes through a formationand the resistivity is measured, changes in the bulk resistivity will bedue to resistivity changes at the bit.

Referring to the schematic showing of FIG. 5, an upper power and controlsub-assembly 36 having one or more current 37 and guard 38 electrodes ismounted on or in and insulated from the drillstring 39. Thissub-assembly also carries a power source 40 and control and switchingelectronics 41. See also driver 41 a for switch arm 41. An insulatedtubular drill collar or gap sub-assembly 42 separates the upper powerand control sub-assembly from the motor housing or lower metallic drillcollars 43.

A resistivity-at-bit lower sub-assembly capable of azimuthalmeasurements is housed by a tubular mandrel 44 extending downwardly fromthe motor 43. This mandrel carries an instrumentation package directlyabove the bit 45. The instrument package comprises a set of one or moreguarded or unguarded current electrodes 46 mounted on and insulated fromthe mandrel or drill collar; and a means 48 a is provided for connectinglower extent of the wireline 48 to the current electrodes 46individually, or in combination, at each level. Each electrode is shownas surrounded by an insulated guard electrode 47 and associatedelectronics to provide focusing and to reduce return currents along themotor housing or drill collar. Accordingly, electrical field “lines” canbe established at different azimuthal locations about the string axis.

Multiple voltage sensing electrodes 49 are mounted on insulated pads 50on the mandrel. The potential difference between the various voltagesensors is selected from the upper control sub-assembly via wirelineconnections 48 from the upper sub-assembly electrodes to the bit-boxelectrodes through the drill collars and/or motor housing. FIG. 5 alsorepresents the combined use of MWD (measure while drilling) technique,together with one of multiple electrodes, as referred to, to measureformation properties. Measured voltage or current values are eitherinterpreted as formation resistivity or IP at control sub-assembly fortransmission to the surface by the methods described in the previousparagraph, or the values themselves are transmitted to the surface forinterpretation. In this case, the results of formation evaluation areequivalent to sensor output.

By proper configuration of insulated drill collars or gapsub-assemblies, electrodes, and wireline connections, a unique boreholeapplication of the surface geophysical dipole-dipole resistivitytechnique is possible. FIG. 6 schematically illustrates thisconfiguration. Other similar configurations are possible correspondingto the various electrode configurations developed for (surface)resistivity and IP measurements. Using this configuration, one or moregap sub-assemblies and wireline system components are used to provideformation resistivity measurements at distances from the boreholepreviously unobtainable by the prior art.

In FIG. 6, a series of insulated, tubular drill collars or gapsub-assemblies 57, and electrically conducting drill collars or sectionsof drillstring 58 and 59 are connected in a dipole-dipole configuration,in accordance with known surface geophysics. A voltage is applied viasource 82 by conductor means 80 and connection means 58 a and 58 bacross conducting sections 58 and 59, which act as effective currentelectrodes.

Electric current 84 is thereby driven from the conducting sections intothe formation 85 surrounding the borehole 85 a. Receiver means 83 iselectrically connected to conducting sections 60 and 61 by conductormeans 81, and connection means 60 a and 60 b, and the receiver meansdetects the potential difference between such conducting sections, whichact as effective potential electrodes. By interpretive means known inthe art of surface geophysics, the electrical resistivity of theformation surrounding the borehole can be determined from such receivermeasurements and knowledge of the voltage at source 82.

In FIG. 7, the apparatus is configured so as to provide measurement ofvariable azimuthal resistivity in the formation adjacent to thedrillstring. A power source at 68 a and suitably driven switchingcircuits at 67 and 71 drive current along paths 77 into and in theformation, through electrodes 65 and 73, located around thecircumference of upper and lower sub-assemblies 64 and 72, mountedbetween upper and lower sections of the drillstring 63 and 63 a, andconnected to the power source by an insulated wireline 70. An insulated,intermediate section of the string appears at 69.

A downhole motor appears above the drill bit 75 at 76. The current flowat electrodes 65 and 73 may be focused by guard electrodes at 74 and 66.Switches 67 and 71 operate to azimuthally distribute the voltageapplication to upper and lower electrodes at different azimuthlocations. Such switches are programmably driven, as at 67 a and 71 a.Multiple voltage-sensing electrodes 81, 82, 83 and 84 are mounted on thecircumference of lower sub-assembly 72. Potential differences betweenvarious voltage sensors are selected by the upper control sub-assemblyvia wireline connection 70. In a manner similar to operation ofapparatus described and shown in FIG. 5, azimuthal resistivity valuesadjacent to the borehole are interpreted and transmitted to the surface.

Referring to FIG. 8, the elements of the invention are shown in moredetail, in association with a drillstring in a well. The string includesmetallic drill pipe, with sections 104 extending from the earth surfacedownwardly in a borehole 120, to connect at 121 to the upper end ofinsulated collar 106. Metallic drillstring section 105 is connected at122 to the lower end of collar 106, and extends downwardly toward adrill bit not shown. The non-conductive portion of collar 106 mayconsist of very high-strength composite material, such as KEVLAR, orglass fibers in resin.

String components 121 and 122 are metallic components of collar 106having pin and box connection to the drill pipe section, and tapered orconical bonded connections to the non-conductive portion of collar 106at 126 and 127. Drilling fluid typically flows downwardly in the stringand through bore 128 in 106; and flows upwardly about the string tocarry borehole cuttings to the surface.

A battery pack (source of voltage) 130 is typically located in hangingsub-assembly 135 above 106, one terminal of the source of voltage inelectrical connection with centralizer (belly-type) springs 132 locatedbetween the battery pack housings 130 and the bore 133 of 104. Anelectrical connection is thereby established to the upper string section104. Hanging sub-assembly 135 supports pack 130 in position, as shown,and may be of any suitable form. Note hang support location 135 c.

Wireline 138 extends downwardly from the battery pack, through theinsulating collar 106 to connect to pulser means 140 a in the lowerdrillstring section. That pulser means is electrically connected tocentralizer (belly-type) springs 141 contacting the bore 142 of lowerstring section 105. Accordingly, the drillstring sections 104 and 105near the collar 106 act as effective upper and lower electrodes, one topass current into the formation, and the other to receive current flowback from the formation.

A second battery pack and housing 140 b supplies power to pulser means140 a and sensor means 140 c. The latter means 140 c produces signalswhich are encoded by pulser means 140 a. A hang support at 140 d carries140 b.

Details of the mechanical positioning of the switching and sensormodules is shown in FIG. 9. A modulator means housing in pressure barrel320 controls flow of electrical current through wireline 6 to thedrillstring 5 by means of en electrical connection from the modulatorhousing to a pressure barrel 320, and from that pressure barrel to thedrillstring by electrically conductive drilling fluids or centralizermeans 322. Signals from the sensor package, housed in pressure barrel323, are carried by line or cable means 325 to a multiplexer meanshoused in barrel 320, and from there to modulator means also housed inbarrel 320. Power is supplied from source housed in pressure barrel 324to the sensors by means 328, to the multiplexer and to the modulator bymeans 327. The entire assembly is supported by hanging sub-assembly 135a carried by the string, and constrained from rotation by means 135 b.

The transceiver/sensor package is shown in its functional relation tothe drillstring in FIG. 10. An insulated wireline 6 is connected formone terminal of a source of voltage or current 24 to the conductivestring section at the lower end of a resistive section of thedrillstring shown schematically at 303. The other terminal of saidsource is connected to the conductive string section at the upper end ofsaid resistive section. A means 309 for modulating or reversing polarityof the source 24 in response to the output of sensor 307 a is provided.The multiple sensor outputs 1 through “n” are combined by a multiplexer307 b before input to the modulator 309.

The apparatus may also be configured in a manner such that the wellborecasing enhances the conductive path for transmitted currents to thesurface. In this configuration, an insulating section is provided in thewellbore casing as shown in FIG. 11. Insulating section 350 confines theflow of electrical currents from the section of drillstring 351 abovethe transmitting gap to the wellbore casing 352 above the insulatingsection 350, thereby increasing the current flow 353 between receiverelectrodes 9 and 9 a proximate the surface. Note connection of surfaceline 8 a to the casing at 9 b.

Other configurations of drillstring and wellbore casing gaps andwireline connections are possible, all with the purpose of improvingsignal strength at the receiver electrodes.

Multiple, non-conducting sub-assemblies may be connected in series, orparallel, or any combination thereof, by use of switchingsub-assemblies, as shown in FIG. 12. A power source 401 is connected ineither positive or negative polarity by switching means 402 to a pair ofconductors 403 and 404 insulated from the drillstring and drillingfluids by tubular sheaths 405 and 406. These conductors may be comprisedor specially designed insulated wireline components. In this form, thedrillstring is comprised of multiple, non-conducting sub-assemblies 407and 409, which are series separated by one or more electricallyconducting 408 and 410. Connector elements 411 and housing 412 areprovided, whereby the conductors are connected to connector elementswhich connect 413 to electrically conducting drillstring elements 408.By appropriate selection of elements 411 to provide connection ornon-connection of the conductors to the electrically conductivedrillstring elements, the non-conducting sub-assemblies are connected inseries, parallel or any combination thereof with the power source.

As in previously described forms of the invention, a modular 414 isdeployed in the bottom hole assembly 415 so as to modulate the flow ofelectric current in the aforementioned circuit for the purpose oftransmission of signals derived from one or more sensors 416.

Referring to FIG. 13, elements of the apparatus are shown in moredetail, in association with a drillstring in a well. The string includesdrill pipe sections, with sections 104 extending from the earth surfacein a borehole 120, to connect at 121 to conductive adapter 435 at theupper end of insulating portion 432 of a non-conductive collar.

The gap sub-assembly may be provided with a resistive element 431providing a leakage path for wireline communication with the bottom holeassembly.

The resistive element 431 is embedded in the insulative material 432 ofthe gap sub-assembly and electrically connected to upper 435 and lower436 conductive fittings at 433 and 434, respectively. Communication fromthe surface to the sensor and modulator electronics is accomplished by acommunications path employing wireline means 437 connected through upperbattery pack 439, to insulated wireline 440, to downhole modulator andsensor electronics 442.

In another form of the invention, the insulated wireline components arereplaced by a conductor 440 within an insulating tubular sheath 441, asshown in FIG. 12.

Pressure changes or flow of drilling fluid may be encoded forcommunication from the surface to downhole components of the invention.FIG. 14 shows the use of a pressure switch 701 for this purpose. Changesin pressure or flow rate of drilling fluid 702 internal to drillstring703 is sensed by pressure switch means 701, which in turn provides inputsignals to control means 704. Control means 704 is used to controloperation of downhole instrumentation, including modulator means 705,power source 706, and sensor means 707. Typically changes in thedrilling fluid flow rate, controlled from the surface, can be used toconserve downhole power consumption by the means of the invention.

In another form of the invention, multiple receiver electrodes 501, 502,503, 504 and 505 are deployed as shown in FIG. 15. Some of theelectrodes may be effected by direct connections 501 a and 505 a, to theactive drillstring or casing 501, or adjacent well casings 505. By aswitching means 506 and comparator means 507, electrode signals arecombined in a manner which provides the best signal reception from adownhole transmitter. The switching and comparator means may also beused to provide information on lateral changes in geologic formation,such as the change in resistivity from formation 508 to formation 509.

The invention improves methods of downhole target detection, location,and tracking while drilling as by means shown in FIG. 16. A time-varyingcurrent 521 is injected along the drillstring and into the formationsurrounding the drillstring by transmitter means 522. Target casing 523provides an electrically conductive path in the formation for currents521. As a result, current is concentrated, 524, on target casing 523.Current flow 524 results in a time varying magnetic field 525, which ismeasured by magnetometer means 526. Time varying magnetic fields 525,measured by means 526 in the bottom hole assembly, bears a knownrelation to the position of target casing 523. Such measurements aretransmitted to the surface for reception by receiver means 9 andcalculation of target position by surface means 528.

The invention also incorporates several additional improvements over theprior art. These are:

1) A means for the generation of low voltage electrical pulses to carrythe signal information and thereby reduce the danger of electricalbreakdown and discharge in the wellbore. In the prior art of directcoupled systems, the impedance mismatch between the source andsurrounding formation was sometimes overcome by generating extremelyhigh voltage pulses by the charging of a downhole capacitor. By reducingthe required voltage, the present novel configuration reduces the hazardof such wellbore discharges.

2) The generation of easily controlled and synthesized low voltage pulsewaveforms also permits the application of recent advances in digitalsignal processing to the detection of low-level signals in the presenceof natural and man-made noise.

3) The improved detection of synthesized waveforms permits waveletsignal processing for the interpretation of low level signals. Waveletanalysis is a relatively new method of signal processing, which permitsefficient “de-noising” of broad-band signals (see Daubechies, I, 1992,“Ten Lectures on Wavelets”, Society for Industrial and AppliedMathematics). The received waveform of a doublet (positive-negativepulse pair) when transmitted through the drillstring-formation path ismodified so as to resemble one of the Daubechies family of wavelets.This permits the compact and therefore fast recognition of electricfield signals in the presence of noise.

4) Detecting the arrival time of electric field pulses generated at thedownhole gap sub-assembly permits interpretation of pulse waveforms inthe time domain, thus allowing determination of distance todiscontinuities in formation resistivity.

5) Improved detection by employing multiple voltage-sensing electrodeson the surface and using common mode rejection and noise cancellationtechniques at the surface receiver allows selection of the bestelectrode combination. The choice of surface electrode combinations maychange during the drilling operation. These changes may be due tochanges in the noise sources, changes in the spatial location of thedownhole transmitter, or changes in the intervening formations.

5a) Improved signal transmission to the surface by optimal selection ofdownhole transmitter locations and combinations and surface potentialsensing electrodes, locations and combinations.

6) A means for changing the carrier frequency using the talk-downcapability to obtain an optimum frequency for the current drilling depthis attainable. On occasion, it may be desirable to use a modulatedsignal carrier frequency rather than pulse transmission.

Theoretical studies indicate that an optimum transmission frequencyexists for different combinations of geologic factors.

7) The invention contemplates a system, the components of which may bedeployed in various ways, according to the requirement sat the wellsite.For example, as an alternative to the configuration shown in FIG. 2b, asa highly resistive formation is penetrated during drilling, it may beuseful to change the bottom hole assembly from an insulated gapconfiguration to a long wireline-direct drillstring connectionconfiguration.

8) The invention contemplates provision of an apparatus for downholeelectric-field telemetry comprising a source of pulsed or amplitudemodulated voltage or current, one or several insulating drill collars,conventional drill collars or gap sub-assemblies, and a system ofinsulated wireline components used to provide electrical connections,insulated from drilling fluids, between the ends of the one or moreaforementioned insulated drill collars in the drillstring.

Such apparatus may be used to optimize the downhole position or depth ina drillhole of a source of pulsed or amplitude modulated voltage orcurrent, by selection of any single or combination of insulated drillcollars or gap sub-assemblies in the drillstring.

In such apparatus, the frequency, waveshape or encoding mechanism of thetransmission system is typically adaptively varied to obtain optimumtransmission characteristics for either or both telemetry and evaluationof formation resistivity and induced polarization characteristics.

9) The apparatus may include two or more surface electric potentialelectrodes connected to a central control unit to adaptively optimizeelectrode location during drilling operations for the purpose ofrejecting common mode and local noise or evaluating geologic structure.One or more of such electrodes is or are either the active drillstringor nearby well casings.

In operation, the formation resistivity and induced polarization, bothat the bit and/or surrounding the borehole, are measured with the sameapparatus and concurrently with borehole telemetry transmissions.

10) The apparatus improves downhole reception of surface-generatedelectric fields by use of multiple surface transmitter electrodesconnected in a configuration to optimize transmission to a downholereceiver.

Such apparatus measures the electric fields in a drillhole through useof insulating drill collars connected by wireline components. Directconnection to the drillstring using widely spaced electrodes andwireline components can be substituted for the aforementioned insulatingdrill collars or gap sub-assemblies. Also, direct connection to thecasing of a well can be substituted for the aforementioned directconnection to the drillstring.

11) The herein described method for the measurement of azimuthal oraverage values of formation resistivity and/or induced polarization mayinclude use of any of, or any combination of, apparatus or devices asreferred to, together with well known geophysical techniques, formeasurement of resistivity and induced polarization.

12) The herein described method for downhole telemetry in producingwells may include apparatus as referred to, together with downholesensors, encoders, and transmission electronics.

More specifically, apparatus to measure azimuthal or average values ofresistivity and induced polarization of the geologic formationsurrounding a drillhole near the bit, typically comprises multiplecurrent electrodes and voltage-sensing electrodes, placed on a mandrelor drill collar just above the bit, and below the motor or other drillcollars, connected by wireline to a set of current electrodes above andseparated from the motor housing or drill collars by an insulating drillcollar or gap sub-assembly. A means for determining toolface direction,such as a pair of cross-axis accelerometers or magnetometers, or otherphysical measurements, may be used to resolve the azimuthal direction ofresistivity or induced polarization measurements.

13) An apparatus and method for detecting and locating a nearbyelectrically conductive target, such as a nearby well casing, mayinclude apparatus as described to inject electric current into theformation surrounding the wellbore and measurement, and analysis of theanomalous vector magnetic fields produced by the concentration of theaforementioned electric current on the target.

The apparatus and methods may be used to detect and/or locate changes information resistivity, due to the presence of an electrically conductiveobject, such as a nearby well casing.

14) The apparatus and methods may be used to locate the position andorientation of a nearby electrically conductive object, such as a wellcasing. See the casing 300 in FIGS. 1, 4 and 7, the presence of whichaffects the return current flow in the formation, to be detected as byvoltage variation detector at 8 at the surface (see FIG. 2). Also,wavelet signal processing may be used to detect anomalous magnetic orelectric-fields. The frequency of a periodic source voltage at theinsulated gap may be varied to obtain maximum electric or magnetic fieldresponse from the conductive target.

15) The electrical and induced potential structure of the formationsurrounding the borehole and of the formation between the surface anddownhole locations can be determined with the apparatus of the inventionby measuring the potential between various of the multiple surfaceelectrodes of the apparatus in response to a known current or voltagewaveform transmitted by the downhole source apparatus, either expresslyfor the purpose of determining the geoelectrical structure or inassociation with telemetry transmission.

Conversely, the apparatus can be used to evaluate electrical and inducedpotential structure of the formation surrounding the borehole and of theformation between the surface and downhole locations by comparison ofvoltage received at various downhole locations in response to knownvoltage or current waveforms generated between various configurations ofsurface electrodes.

16) An apparatus and method for downhole magnetometric formationevaluation. By addition of appropriate magnetic field sensors to thebottom hole assembly, time varying magnetic fields produced by theconcentrated flow of electric current in electrically conductive regionsof the formation can be detected. Using the prior art of surfacegeophysics, the electrical structure of the formation surrounding theborehole is determined.

Various uses of the invention are listed as follows:

1. Use of the bottom hole assembly below a non-conducting drill collar,as an electrode for transmission of electric currents in anelectric-field borehole telemetry system, the non-conducting drillcollar providing an insulating gap for transmission of electric currentsto the surface.

2. Use of centralizers as electrical contactors between components of anelectric-field telemetry system mounted in a drillstring and thedrillstring itself, the bow strings of the centralizers making contactwith the interior wall of the drillstring.

3. Use of drillstring stabilizers as electrical between drillstringcomponents and the borehole wall in an electric-field telemetry system,the stabilizer blades making electrical contact with the borehole wall.

4. Use of drill collars comprised of electrically insulating material toprovide electrical gaps in the drillstring, said gaps being sufficientlylonger than in the prior art, for the purpose of reducing downhole powerrequirements in an electric-field downhole telemetry system.

5. Use of one or more electrically insulating drillstring collars in anelectrically conductive drillstring, together with one or moreelectrically insulated sections of wellbore casing, the ends of theinsulating drillstring collars electrically connected by insulatedwireline components and the insulated sections of wellbore casinglocated, so as to maximize the flow of electric current to the surfacein an electric-field downhole telemetry system.

6. Use of one or more electrically insulating drillstring collars, theends of the insulating collars connected by electrically insulatedwireline components in a manner such that the impedance of the entireassembly, measured from the surface of the earth, is carried so as tocomprise a borehole telemetry system.

7. Use of a downhole pressure switch in an electric-field telemetrysystem to detect acoustic pulses, transmitted form the surface, tocontrol operation of the electric-field telemetry system.

FIG. 17 is a schematic showing a pipe string 299 having multipleinsulated, sub-surface pipe string sections 301-304, across whichinstrumentation or circuitry 305 and 306 in upper and lower housings 307and 308 is connected. See connection 309 from circuitry orinstrumentation 305 in upper housing 307 to the bore of string section301; and see connection 311 from circuitry or instrumentation 306 inlower housing 308 to the bore of string section 304. Additionalconnections are shown at 313, 314 and 315 from circuitry 205 to thestring sections 302 to 304. Wirelines are indicated at 321-323. Suchinstrumentation may include batteries, pulse producing means, andcircuitry such as amplifiers, and pulse wave shaping equipment, encodingequipment, and frequency and phase shifting means.

FIG. 18 shows multiple electrodes, including surface electrodes 330 and331 spaced at distances d₁ and d₂ from the top of the sub-surface pipestring 333. The latter is representative of any of the pipe stringsdescribed above and shown in any of FIGS. 1-16, containing apparatus asdescribed above and shown in any of FIGS. 1-16. Such electrodes aretypically on or under the ground surface, and adapted to sense changesin electromagnetic fields including electrical fields transmitted in theunderground formation and in the pipe string, to the surface, byoperation of the down-hole equipment including pulse producingapparatus; and such electrodes and/or the pipe string are also adaptedto transmit control signals from the surface to the sub-surfaceequipment in the pipe string. Signal processing means, such as acomputer, is shown at 340, suitably connected at 341, 342, 343 and 344with the electrodes and the pipe string, as via amplifier and/or filterequipment 345. Upper and lower instrument housings in the pipe stringare indicated at 346 and 347. A bit box appears at 348. The location ofa target underground steel casing 349 may be detected, as describedabove. Means 370 may be provided to shift the positions of theelectrodes, in relation to the underground formation, to enhanceidentification of underground formation characteristics.

The underground instrumentation in the pipe string including pulseproducing means, is capable of producing short duration pulse wave formsselected to obtain optimum or near optimum transmission of electricfield change characteristics in the underground formation. Suchinstrumentation includes means for producing pulse polarity reversal;for producing pulse waveforms of less than 200 ms duration; and forproducing such waveforms and characterized by polarity reversal, suchwaveform may be of digital type. The pulser means is typically operatedat a substantial distance above the drill bit. The undergroundinstrumentation may also include a receiver for receiving signalstransmitted downwardly from the surface, for use in controllingequipment such as drilling equipment, or controlling the instrumentationin the upper and lower housings, (switching battery connections, andcontrolling pulser operation).

What is claimed is:
 1. An apparatus for borehole electric fieldtelemetry comprising a source of modulated voltage or current, at leastone axially extending insulative collar connected between pipe sectionsin a pipe string, and a system of insulated wireline componentsproviding electrical connections, insulated form drilling fluids,between the ends of the one or more aforementioned insulative collars inthe pipe string, to transmit said voltage or current, said source ofmodulated voltage or current comprising electrical pulse-producing meansfor producing means for producing short duration pulse wave formsselected to obtain optimum transmission characteristics in theunderground formation, said electrical connections being to the drillstring, and there being upper and lower instrument housings associatedwith said electrical connections which are upper and lower connections,said housings supported within the pipe string, the upper housinglocated above at least one of said insulative collars, and the lowerhousing projecting below said insulative collar, said pulse-producingmeans located within at least one of said housings, and includingmultiple electrodes electrically connected to a signal processor andoperable to receive transmission of a pulse wave form in the undergroundformation.
 2. The apparatus of claim 1 including vertically spacedhousing components in the string for said first and secondinstrumentations.
 3. The apparatus of claim 1 wherein said housings arelocated within at least one of the electrically conductive sections ofthe pipe string.
 4. The apparatus of claim 1 wherein said pipe string isa drill pipe string.
 5. The apparatus of claim 1 including surfaceelectrode means to detect changes in said electric field in theformation below said surfaces, said surface electrode means located atthe upper top level of the pipe string.
 6. The apparatus of claim 5wherein said surface electrode means include a primary electrodeproximate the pipe string at the surface, and a secondary electrode inthe formation and spaced away from the pipe string.
 7. The apparatus ofclaim 1 wherein said voltage source includes circuitry to produce aseries of short duration pulses.
 8. The apparatus of claim 1 wherein thestring is drillstring in which a drill motor and bit box, are carried.